The present invention relates to an information recording method and apparatus for recording information on a recording medium and reproducing the recorded information, and more particularly to such an information recording method and apparatus for effectively reducing variation of an average DC level of recorded data or a reproduced signal occurring when information is recorded on or reproduced from the recording medium, such as an optical disk medium, by using a coding rule without a DC free property.
Generally, in an information recording system in an optical disk file, it is effective to improve the signal-to-noise ratio of the reproduced data in order to reduce the error rate of the reproduced data.
Heretofore, as a method of binarizing the reproduced signal, the following two methods are known. The first method involves binarizing by a fixed slice level a detection signal that is varied in accordance with the presence of a record mark recorded on an optical disk, and is hereinafter referred to as an original waveform detection method. The second method involves detecting a position for a code word by differentiating the detection signal, and is hereinafter referred to as a differential detection method. In a system for causing the code word to correspond to the center of the record mark, hereinafter referred to as a mark position recording system, the detection signal is conventionally first-order differentiated, and a zero-cross point of the differentiated signal is detected. In a system for causing the code word to correspond to both ends of the record mark, hereinafter referred to as a mark edge recording system, the detected signal is conventionally second-order differentiated and a zero-cross point of the differentiated signal is detected.
The degree of reduction of the signal-to-noise ratio occurring in binarizing the reproduced signal is smaller in the original waveform detection method than in the differential detection method.
On the other hand, the DC free property may be useful for characterizing the modulation system (coding rule). The average DC level (DC component) of the reproduced signal obtained from the record pattern is always constant within the range of a certain data length (for example, one byte period). One method of estimating the degree of the DC free property uses an accumulated charge value. The accumulated charge value is an accumulated value in a certain data period when the polarity corresponding to the record mark or the side of the mark of a reproduced waveform obtained from the record mark is + (plus) and the polarity corresponding to an unrecorded portion between the marks, that is, a gap, is - (minus). When the DC free property is perfectly satisfied, the accumulated charge value is always 0. Generally, when the accumulated charge value in a proper section of the data length, for example, one byte, is 0, it is judged that the record pattern is DC free. The accumulated charge value is also known as the DSV (Digital Sum Variation).
No variation of the average DC level of the reproduced signal occurs by using the coding rule having the DC free property even when the reproduced signal system is connected by means of a coupling capacitance. In the differential detection method, since signals of the lower side and the upper side which are equal to each other in number and have the same amplitude are produced from the nature of the differential waveform, the average DC level of the reproduced signal is maintained substantially constant even when there is no DC free property in the modulation system itself.
In the standard of the ISO (International Standardization Organization), the mark position recording system of the 2-7 RLL (Run Length Limited) is used as the modulation system and a re-synchronous pattern of one byte is inserted at intervals of 20 bytes of data. The re-synchronous pattern (RESYNC) has a function of correcting a shift when a phase relation of a bit extraction clock produced by a PLL and the reproduced data is shifted by one bit or more.
In the ISO standard, the re-synchronous pattern is a specific pattern and is not changed by data positioned before and behind it. Further, since the mark position recording system is used as the recording system, detection of the mark position is made by the usual differential detection method, and accordingly the need of the DC free property is not so strong.